copyright © 1998 by addison wesley longman, inc. 1 chapter 3 syntax - the form or structure of the...

Copyright © 1998 by Addison Wesley Longman, Inc. 1

Chapter 3Syntax - the form or structure of the expressions,

statements, and program units

Semantics - the meaning of the expressions, statements, and program units

Who must use language definitions? 1. Other language designers 2. Implementors 3. Programmers (the users of the language)

A sentence is a string of characters over some alphabetA language is a set of sentencesA lexeme is the lowest level syntactic unit of a language (e.g., *, sum, begin)A token is a category of lexemes (e.g., identifier)

Formal approaches to describing syntax:

1. Recognizers - used in compilers 2. Generators - what we'll study


Chapter 3Context-Free Grammars

- Developed by Noam Chomsky in the mid-1950s - Language generators, meant to describe the syntax of natural languages

- Define a class of languages called context-free languages

Backus Normal Form (1959) - Invented by John Backus to describe Algol 58 - BNF is equivalent to context-free grammars

A metalanguage is a language used to describe another language.

In BNF, abstractions are used to represent classes of syntactic structures--they act like syntactic variables (also called nonterminal symbols)

e.g.

<while_stmt> -> while <logic_expr> do <stmt>

This is a rule; it describes the structure of a while statement


Chapter 3A rule has a left-hand side (LHS) and a right-hand

side (RHS), and consists of terminal and nonterminal symbols

A grammar is a finite nonempty set of rules

An abstraction (or nonterminal symbol) can have more than one RHS

<stmt> -> <single_stmt> | begin <stmt_list> end

Syntactic lists are described in BNF using recursion

<ident_list> -> ident | ident, <ident_list>

A derivation is a repeated application of rules, starting with the start symbol and ending with a sentence (all terminal symbols)


Chapter 3An example grammar:

<program> -> <stmts><stmts> -> <stmt> | <stmt> ; <stmts><stmt> -> <var> = <expr><var> -> a | b | c | d<expr> -> <term> + <term> | <term> - <term><term> -> <var> | const

An example derivation:

<program> => <stmts> => <stmt> => <var> = <expr> => a = <expr> => a = <term> + <term> => a = <var> + <term> => a = b + <term> => a = b + const

Every string of symbols in the derivation is a sentential formA sentence is a sentential form that has only terminal symbolsA leftmost derivation is one in which the leftmost nonterminal in each sentential form is the one that is expandedA derivation may be neither leftmost nor rightmost


Chapter 3

<program>

<stmts>

<stmt>

<var> = <expr>

a <term> + <term>

<var> const

b

A grammar is ambiguous iff it generates a sentential form that has two or more distinct parse trees

A parse tree is a hierarchical representation of a derivation


Chapter 3An ambiguous expression grammar:

<expr> -> <expr> <op> <expr> | const<op> -> / | -

<expr> <expr>

<expr> <op> <expr> <expr> <op> <expr>

<expr><op><expr> <expr><op><expr>

const - const / const const - const / const

If we use the parse tree to indicate precedence levels of the operators, we cannot have ambiguity

An unambiguous expression grammar:<expr> -> <expr> - <term> | <term><term> -> <term> / const | const

<expr>

<expr> - <term>

<term> <term> / const

const const

!


Chapter 3<expr> => <expr> - <term> => <term> - <term>

=> const - <term> => const - <term> / const => const - const / const

Operator associativity can also be indicated by agrammar

<expr> -> <expr> + <expr> | const (ambiguous)

<expr> -> <expr> + const | const (unambiguous)

<expr>

<expr> + const

<expr> + const

const

Extended BNF (just abbreviations):1. Optional parts are placed in brackets ([]) <proc_call> -> ident [ ( <expr_list>)]2. Put alternative parts of RHSs in parentheses and separate them with vertical bars <term> -> <term> (+ | -) const3. Put repetitions (0 or more) in braces ({}) <ident> -> letter {letter | digit}


Chapter 3BNF:

<expr> -> <expr> + <term> | <expr> - <term> | <term><term> -> <term> * <factor> | <term> / <factor> | <factor>

EBNF:

<expr> -> <term> {(+ | -) <term>}<term> -> <factor> {(* | /) <factor>}

Syntax Graphs - put the terminals in circles or ellipses and put the nonterminals in rectangles; connect with lines with arrowheads e.g., Pascal type declarations

type_identifier

( identifier ) ,

constant .. constant


Chapter 3

Recursive Descent Parsing

- Parsing is the process of tracing or constructing a parse tree for a given input string - Parsers usually do not analyze lexemes; that is done by a lexical analyzer, which is called by the parser - A recursive descent parser traces out a parse tree in top-down order; it is a top-down parser - Each nonterminal in the grammar has a subprogram associated with it; the subprogram parses all sentential forms that the nonterminal can generate - The recursive descent parsing subprograms are built directly from the grammar rules - Recursive descent parsers, like other top-down parsers, cannot be built from left-recursive grammars


Chapter 3Example: For the grammar:

<term> -> <factor> {(* | /) <factor>}

We could use the following recursive descent parsing subprogram (this one is written in C)

void term() { factor(); /* parse the first factor*/ while (next_token == ast_code || next_token == slash_code) { lexical(); /* get next token */ factor(); /* parse the next factor */ } }

Static semantics ( have nothing to do with meaning) Categories:

1. Context-free but cumbersome (e.g. type checking) 2. Noncontext-free (e.g. variables must be declared before they are used)


Chapter 3

Attribute Grammars (AGs) (Knuth, 1968)

- Cfgs cannot describe all of the syntax of programming languages - Additions to cfgs to carry some semantic info along through parse trees

Primary value of AGs: 1. Static semantics specification 2. Compiler design(static semantics checking)

Def: An attribute grammar is a cfg G = (S, N, T, P) with the following additions:

1. For each grammar symbol x there is a set A(x) of attribute values

2. Each rule has a set of functions that define certain attributes of the nonterminals in the rule

3. Each rule has a (possibly empty) set of predicates to check for attribute consistency


Chapter 3Let X0 -> X1 ... Xn be a rule.

Functions of the form S(X0) = f(A(X1), ... A(Xn)) define synthesized attributes

Functions of the form I(Xj) = f(A(X0), ... , A(Xn)), for i <= j <= n, define inherited attributes

Initially, there are intrinsic attributes on the leaves

Example: expressions of the form id + id - id's can be either int_type or real_type - types of the two id's must be the same - type of the expression must match it's expected type

BNF:

<expr> -> <var> + <var> <var> -> id

Attributes:

actual_type - synthesized for <var> and <expr> expected_type - inherited for <expr>


Chapter 3Attribute Grammar:

1. Syntax rule: <expr> -> <var>[1] + <var>[2] Semantic rules: <expr>.actual_type <var>[1].actual_type Predicate: <var>[1].actual_type = <var>[2].actual_type <expr>.expected_type = <expr>.actual_type

2. Syntax rule: <var> -> id Semantic rule: <var>.actual_type lookup (id, <var>)

How are attribute values computed?

1. If all attributes were inherited, the tree could be decorated in top-down order.2. If all attributes were synthesized, the tree could be decorated in bottom-up order.3. In many cases, both kinds of attributes are used, and it is some combination of top-down and bottom-up that must be used.


Chapter 31. <expr>.expected_type inherited from parent

2. <var>[1].actual_type lookup (A, <var>[1]) <var>[2].actual_type lookup (B, <var>[2]) <var>[1].actual_type =? <var>[2].actual_type

3. <expr>.actual_type <var>[1].actual_type <expr>.actual_type =? <expr>.expected_type

Dynamic Semantics

- No single widely acceptable notation or formalism for describing semantics

I. Operational Semantics - Describe the meaning of a program by executing its statements on a machine, either simulated or actual. The change in the state of the machine (memory, registers, etc.) defines the meaning of the statement


Chapter 3- To use operational semantics for a high-level

language, a virtual machine in needed

- A hardware pure interpreter would be too expensive

- A software pure interpreter also has problems: 1. The detailed characteristics of the particular computer would make actions difficult to understand 2. Such a semantic definition would be machine- dependent

- A better alternative: A complete computer simulation

- The process: 1. Build a translator (translates source code to the machine code of an idealized computer) 2. Build a simulator for the idealized computer

- Evaluation of operational semantics: - Good if used informally - Extremely complex if used formally (e.g., VDL)


Chapter 3Axiomatic Semantics

- Based on formal logic (first order predicate calculus)- Original purpose: formal program verification- Approach: Define axioms or inference rules for each statement type in the language (to allow transformations of expressions to other expressions)

- The expressions are called assertions

- An assertion before a statement (a precondition) states the relationships and constraints among variables that are true at that point in execution

- An assertion following a statement is a postcondition- A weakest precondition is the least restrictive precondition that will guarantee the postcondition

- Pre-post form: {P} statement {Q}

- An example: a := b + 1 {a > 1} One possible precondition: {b > 10} Weakest precondition: {b > 0}


Chapter 3Program proof process: The postcondition for the

whole program is the desired results. Work back through the program to the first statement. If the precondition on the first statement is the same as the program spec, the program is correct.

- An axiom for assignment statements:

{Qx->E} x := E {Q} - The Rule of Consequence:

{P} S {Q}, P' => P, Q => Q'

{P'} S {Q'}

- An inference rule for sequences - For a sequence S1;S2:

{P1} S1 {P2}{P2} S2 {P3}

the inference rule is:

{P1} S1 {P2}, {P2} S2 {P3}

{P1} S1; S2 {P3}


Chapter 3

- An inference rule for logical pretest loops For the loop construct:

{P} while B do S end {Q}

the inference rule is:

(I and B) S {I}

{I} while B do S {I and (not B)}

where I is the loop invariant.

Characteristics of the loop invariant

I must meet the following conditions:

1. P => I (the loop invariant must be true initially) 2. {I} B {I} (evaluation of the Boolean must not change the validity of I) 3. {I and B} S {I} (I is not changed by executing the body of the loop) 4. (I and (not B)) => Q (if I is true and B is false, Q is implied) 5. The loop terminates (this can be difficult to prove)


Chapter 3- The loop invariant I is a weakened version of the

loop postcondition, and it is also a precondition.

- I must be weak enough to be satisfied prior to the beginning of the loop, but when combined with the loop exit condition, it must be strong enough to force the truth of the postcondition

- Evaluation of axiomatic semantics:

1. Developing axioms or inference rules for all of the statements in a language is difficult

2. It is a good tool for correctness proofs, and an excellent framework for reasoning about programs, but it is not as useful for language users and compiler writers

Denotational Semantics

- Based on recursive function theory- The most abstract semantics description method- Originally developed by Scott and Strachey (1970)


Chapter 3- The process of building a denotational spec for a

language: 1. Define a mathematical object for each language entity 2. Define a function that maps instances of the language entities onto instances of the corresponding mathematical objects

- The meaning of language constructs are defined by only the values of the program's variables

- The difference between denotational and operational semantics: In operational semantics, the state changes are defined by coded algorithms; in denotational semantics, they are defined by rigorous mathematical functions

- The state of a program is the values of all its current variables s = {<i1, v1>, <i2, v2>, …, <in, vn>}

- Let VARMAP be a function that, when given a variable name and a state, returns the current value of the variable VARMAP(ij, s) = vj


Chapter 31. Decimal Numbers

<dec_num> 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | <dec_num> (0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9)

Mdec('0') = 0, Mdec ('1') = 1, …, Mdec ('9') = 9 Mdec (<dec_num> '0') = 10 * Mdec (<dec_num>) Mdec (<dec_num> '1’) = 10 * Mdec (<dec_num>) + 1 … Mdec (<dec_num> '9') = 10 * Mdec (<dec_num>) + 9


Chapter 32. Expressions

Me(<expr>, s) = case <expr> of <dec_num> => Mdec(<dec_num>, s) <var> => if VARMAP(<var>, s) = undef then error else VARMAP(<var>, s) <binary_expr> => if (Me(<binary_expr>.<left_expr>, s) = undef OR Me(<binary_expr>.<right_expr>, s) = undef) then error

else if (<binary_expr>.<operator> = ‘+’ then Me(<binary_expr>.<left_expr>, s) + Me(<binary_expr>.<right_expr>, s) else Me(<binary_expr>.<left_expr>, s) * Me(<binary_expr>.<right_expr>, s)


Chapter 33 Assignment Statements

Ma(x := E, s) = if Me(E, s) = error then error else s’ = {<i1’,v1’>,<i2’,v2’>,...,<in’,vn’>}, where for j = 1, 2, ..., n, vj’ = VARMAP(ij, s) if ij <> x = Me(E, s) if ij = x

4 Logical Pretest Loops

Ml(while B do L, s) = if Mb(B, s) = undef then error else if Mb(B, s) = false then s else if Msl(L, s) = error then error else Ml(while B do L, Msl(L, s))


Chapter 3- The meaning of the loop is the value of the

program variables after the statements in the loop have been executed the prescribed number of times, assuming there have been no errors

- In essence, the loop has been converted from iteration to recursion, where the recursive control is mathematically defined by other recursive state mapping functions

- Recursion, when compared to iteration, is easier to describe with mathematical rigor

Evaluation of denotational semantics:

- Can be used to prove the correctness of programs

- Provides a rigorous way to think about programs

- Can be an aid to language design

- Has been used in compiler generation systems

copyright © 1998 by addison wesley longman, inc. 1 chapter 3 syntax - the form or structure of the...

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